Question 6A331

When the field winding of a shunt-wound DC motor opens, the magnetic flux produced by the field essentially drops to zero (or near zero due to residual magnetism). Motor speed is inversely proportional to the magnetic flux. With practically no field flux, the motor must spin at an extremely high speed to generate enough back electromotive force (EMF) to counteract the applied armature voltage. Under no-load conditions, there is very little opposing torque, allowing the motor to accelerate rapidly and uncontrollably. This phenomenon is known as "racing" or "runaway." At excessive speeds, the centrifugal forces can become so great that the armature windings or other mechanical components may fly apart, destroying the motor.